The hyphen in "one-time winner" is used because:

The hyphen in "one-time winner" is used because one and time modify winner.

Those two words both describe the word winner, which is why it is necessary to hyphen them in order to sort of make one single word out of those two. One-time, when hyphenated, is an adjective used to modify the noun following it.

Thy Hyphen is a punctuation used to join to join words. In this case "One-time" winner